At age 22, fourth-grade teacher Jessica King is already a charity veteran.

The recent University of Pennsylvania graduate, with a degree in civic communications, started volunteering when she was 15, as a swimming teacher to special education students.

As an undergraduate, King and her friends mentored children in West Philadelphia through the Netter Center for Community Partnerships. She eventually became the student director of the mentoring program, overseeing about 300 mentors.

Her first full-time job is teaching children in a charter school in Camden, New Jersey, an area with similar challenges to West Philly.

"Checks can be written and buttons can be pressed online, but giving a week's worth of food to someone that you packaged up yourself, that is a different kind of human connection," King said.

Passion is the top reason why millennials, aged 18 to 34, support a charitable cause, a recent study by insurer Country Financial found. Favorite charities include those that affect a family member or friend, or have strong community ties.

King agreed, saying she wants a career that allows her to help others, even if it is not lucrative, and teaching is an extension of her belief in giving back to the community.

Millennials prefer to use their personal skills when giving to charity, the 2015 Millennial Impact Report, conducted by research agency Achieve, showed. Seventy-seven percent of millennial employees said they are more likely to volunteer if they can use a skill or expertise to benefit the cause.

"When a millennial gives an asset of any kind, including time, skills, networks and dollars, they view their assets as equal (value)," said Derrick Feldmann, president of Achieve and lead researcher of the Millennial Impact Project.

Sixty-five percent of millennials are likely to volunteer if a co-worker participates in a charity, compared with 44 percent if a supervisor does, the 2015 Millennial Impact report found.

For many millennials, time is the easiest thing to give because their money is limited: only 21 percent plan to give more to charity during the holiday season, according to Country Financial.

Grad student Jenna Moss, 30, for example, has helped coach and do mock interviews with unemployed adults looking to get back into the workforce in New York City. Before returning to school for her MBA, she worked for a non-profit in the art world.

Crowdfunding sites like Kickstarter offer the most valuable way for millennials to channel their charitable giving, but social media campaigns like the ones on Facebook have limits, Moss said.

"It creates awareness and a community, but that only goes so far. At the end of the day, for example, Planned Parenthood needs money, not just pink images."